thank you for posting all your specs! kudos!
please describe the problem in a little more detail: "no one is in your session" ... does this happen on some calls, a call or two a day, every call immediately which means you can't even use the system, or did it only happen twice last year and you're just now mentioning it to us as a quick note in passing?
I am going to "assume" that it happens a lot when your system is fairly well loaded and not too often otherwise.
I am not going to assume that your agents are successfully logging in and hearing "you are the only person in this conference" upon answering their phones.
So the question: are your agents on the same physical network as your dialer? are your agents phones successfully registering to asterisk? are your agents phones ringing when they check in? do they hear "you are the only person in this conference"?
PS: Ordinarily this problem is the result of a bad network connection to the server from the agents (not enough bandwidth, someone with peer-to-peer software using all the bandwidth, 8 agents watching YouTube, that sort of thing). But often it turns out to just be "not completely set up yet). So You'll Let Me Know where we are.
And again: thanks for posting your specs! I love that.
oops: is this a multiserver installation?